Chrome dyes or acid mordant dyes are acid dyes having additional groups which enable the dye to form a stable co-ordination compound with chromium or with the fibre, thereby improving light and wet fastness. The complex formation is accompanied by bathochromic shift of the colour and the hue may change from red to blue. An example of such a dye is C.I. Mordant Blue 79, when applied as a levelling acid dye it produces a bright red shade, but as an afterchrome dye it produces a deep blue shade. The red acid dye has poor wet fastness, whilst the blue chromed dye has exceptionally high wet fastness. Both dyes give equally level dyeing as they are applied in the same way prior to chroming.

In the unchromed state, the dyes are not fast to alkali. Chrome dyes are not as bright as acid dyes and the shades are similar to 1:2 metal-complex dyes. The latter dyes are suitable for pale to medium shades, while chrome dyes are best for full depths. The light fastness of some chrome dyes is not good in pale depths. The dyes of both dye-classes are often brightened by adding to the recipe a small amount of milling acid dyes.

Chrome dyes have a special position in wool dyeing, since when applied by the afterchrome method, they have very good level-dyeing and migration properties and excellent wet fastness after chroming. Because of their high fastness performance and economy, chrome dyes are most widely used for heavy shades, such as navy and black, and their good level-dyeing properties make them suitable for all substrates.

Some important chrome dyes used for specific shade areas are as follows :

The disadvantages of chrome dyes include long dyeing times, potentially high level of fibre damage, severe changes in colour during chroming, difficulties in correction of faulty dyeing and chromium residues in the effluent. The disadvantages may eventually lead to ceasing the use of chrome dyes in near future.

Some chrome dyes have poor aqueous solubility and may be deposited on the surface of the dyed material in the presence of hard water and acid. Francolane (Francolour) dyes were marketed in finely dispersed free acid form. As the temperature of the dyebath is raised between 70 and 80ºC, the dye gradually dissolves and is adsorbed by the wool to give a level, well-diffused dyeing.

Except for bright shades, chrome dyes are used for all branches of wool dyeing, namely loose wool, rags, slubbing, yarns and pieces, where fastness required is higher than that obtainable with acid dyes. One problem with chrome dyed loose wool is that chromium soaps are formed with free fatty acids in spinning oil, which are difficult to remove by scouring and causes poor fastness to rubbing.

Azo About 80% of chrome dyes belong to azo group, mostly monoazo with a few important disazo dyes. These cover the whole hue range except bright blues, violets and greens. Azo chrome dyes show highest fastness to light and wet treatments. They can be applied by all three mordanting methods.

Anthraquinoid The parent dye in the anthraquinoid group is alizarin or 1,2-dihydroxyanthraquinone. The dye was originally obtained from madder, a natural product, and then manufactured synthetically. It has no affinity for wool and can be applied after mordanting.

Triphenylmethane Triphenylmethane group incorporates important bright blue chrome dye, C.I. Mordant Blue 1 and Violet 3. These dyes are derivatives of salicylic acid. Like all the triphenylmethane dyes, their fastness to light is only moderate.

Xanthene The members belonging to this group are very few, but brightest chrome red, C.I. Mordant Red 27 (9-14), belongs to this class.

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Mazharul Islam Kiron is a textile consultant and researcher on online business promotion. He is working with one European textile machinery company. He is also a contributor of Wikipedia.
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